There is well known a control apparatus for a vehicular automatic transmission configured to selectively establish a plurality of shift positions by engaging respective combinations of frictional coupling devices selected from a plurality of frictional coupling devices, with output hydraulic pressures of respective ones of a plurality of solenoid valves provided in a hydraulic control circuit. The above-indicated hydraulic control circuit has a risk of occurrence of an abnormal output of any solenoid valve, which takes place due to an abnormality of an electronic control device per se, which operates to determine a need for shifting the automatic transmission and generate an electric signal. Such abnormal output of the solenoid valve may cause an engaging action of any irrelevant frictional coupling device other than the frictional coupling devices normally required to be placed in the engaged state for establishing the presently selected shift position of the automatic transmission, namely, any frictional coupling device not normally required to be placed in the engaged state for establishing the selected shift position. In this event, the engaging action of the above-indicated frictional coupling device may cause the automatic transmission to perform an unnecessary shifting action from the presently established shift position [for example, a shifting action causing a comparatively high ratio of a stepping change of the speed ratio of the automatic transmission (stepping ratio=ratio of speed ratios of two shift positions=speed ratio of the lower-speed position/speed ratio of the higher-speed position)], or may cause a partially engaged state (a slipping action) of any one of the frictional coupling devices normally required to be placed in its engaged state, which one frictional coupling device has the relatively smallest torque capacity. Namely, the abnormality of the electronic control device per se may influence a shifting control of the automatic transmission.
In view of a possibility of occurrence of an abnormality of the electronic control device per se, there has been proposed to provide the hydraulic control circuit with a fail-safe valve having a fail-safe function to prevent the engaging action of a frictional coupling device due to an abnormal output of a solenoid valve, by cutting off an oil passage provided to apply the output hydraulic pressure of the abnormal solenoid valve to the frictional coupling device, or to establish a fail-safe shift position so as to reduce the stepping ratio (difference) of the speed ratios, by switching the oil passages utilizing the output hydraulic pressure of the abnormal solenoid valve. On the other hand, it is desirable not to use the fail-safe valve, from the standpoint of needs for reducing the weight and size of the hydraulic control circuit and for reducing the number of the required components or costs. Namely, it is desirable to provide a fail-safe function equivalent to that of the fail-safe valve, without using the fail-safe valve. Patent Document 1 discloses a control apparatus for a transmission configured to perform shifting actions using a hydraulic control circuit not provided with the fail-safe valve, which control apparatus determines, on the basis of amounts of electric current applied to solenoid valves, whether the number of the solenoid valves concurrently placed in the engaged state is larger than in the normal states of the solenoid valves, and cuts off the supply of an electric power to the irrelevant solenoid valve other than the solenoid valves required to be placed in the engaged state, if the number is determined to be larger than in the normal states, so that the control apparatus performs the fail-safe function equivalent to that of the fail-safe valve.